IN MAY 2015, a small unmanned Nasa spacecraft called Dawn entered orbit around Ceres, the largest object in the rubble-strewn asteroid belt that drifts between the orbits of Mars and Jupiter. Planetary scientists are in their element, gathering clues about the origins of the solar system and the processes that make (and occasionally destroy) entire worlds. Ceres has sufficient gravity to have pulled itself into a spherical shape, but it's not a moon. Instead it's classified as a 'dwarf planet'.
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